Due to the worldwide environmental considerations particularly relating to use of energy resources, it has become, recently more important to utilize relatively medium to low temperature heat sources or resources, such as geothermal steam and/or geothermal brine as well as industrial waste heat, for power production.
An Organic Rankine Cycle (ORC) is well suited to exploit the energy content of a medium to low temperature heat source or resource due to the relatively low boiling point of organic motive fluid. Organic fluid flowing in a closed cycle vaporizes after extracting heat from the medium to low temperature heat source or resource. The vapor is expanded in an organic vapor turbine that converts heat in the vapor to work and produces heat-depleted or expanded organic vapor that is condensed in a condenser. The condensed organic fluid is returned to the vaporizer, and the cycle is repeated.
An important consideration in designing the power capacity of such a power plant is the selection of a suitable turbine configuration. Reliable operation of the turbine is contingent upon the structural strength of the shaft that enables turbine rotor rotation and upon the ability of the bearings that support the turbine shaft to absorb both the radial load and axial thrust imposed by the expansion of the motive fluid within the turbine.
One prior art bearing arrangement for supporting a rotating turbine shaft is an overhang design illustrated in FIG. 1. To support the mechanical load imposed by three turbine wheels 2, 3 and 4 which permit the expansion of the working fluid in separate stages carried on turbine shaft 6, two bearings 7 and 8 are provided at the inlet end 5 of turbine shaft 6, which is proximate to the port through which the working fluid is introduced into the turbine interior, and are distant from the outlet end 9 of turbine shaft 6 being close to the port from which the expanded working fluid exits the turbine. The overhang bearing arrangement simplifies assembly and maintenance as both bearings 7 and 8 are at the same side of shaft 6; however, it reduces the maximum mechanical load that can be supported due to the high bending stress that the turbine shaft experiences. Due to the proximity of the two bearings, the moment arm resulting from the weight of the turbine wheels which is applied on the shaft is unidirectional, producing a significantly large moment that causes the shaft to be susceptible to bending.
It is to be noted that the maximum number of turbine wheels that can be supported by the overhang bearing arrangement is usually limited to three as a result of the bending stress, significantly reducing the power output of a turbine from what could be achieved if more turbine wheels could be incorporated therewith.
Another disadvantage of the overhang bearing arrangement is that the end of the turbine shaft that is unsupported by the bearings can undergo an induced vibration phenomena, particularly flexural vibration. Such vibration can result in damage to elements with small radial clearance such as seals.
WO 2013/171685 discloses an ORC system that comprises a radial turbine of the axial inflow and radial outflow type. The turbine is formed by a single rotor disc that carries rotor blades to define a plurality of stages and that is provided with an auxiliary opening between two successive radially spaced stages. The auxiliary opening is interposed between an inlet and an outlet of the turbine, and is in fluid connection with an auxiliary cogeneration circuit so as to extract from the turbine or inject into it organic working fluid at an intermediate pressure between an injection pressure and a discharge pressure. The rotor disc is supported in a casing by two bearings, and is mounted at an end of the shaft that is cantilevered with respect to the casing according to an overhang design.
Since the vapors expand radially outwardly from the turbine shaft in this prior art configuration, large mechanical loads are imposed on the turbine shaft and on the bearings. The various radially spaced stages of the single rotor disc are very closely fitted to the stator blades, and are therefore very sensitive to expansion and contraction forces, particularly due to the application of radial forces to the stages. The applied radial forces increase for the second and third stages, which are more distant from the turbine shaft than the first stage.
Other disadvantages of this single turbine wheel configuration relate to the need of accommodating the large-sized and heavy rotor that has a correspondingly high moment of inertia, requiring an increased torque to drive the rotor, and also to the pressure losses of the vapor exiting the turbine via a 90-degree turn through a volute.
Another prior art bearing arrangement for supporting a turbine shaft is the rotor between bearings design where two bearings are axially spaced. Although the level of bending stress and vibrations is significantly reduced relative to the overhang bearing arrangement by virtue of the axially spaced bearings, one or both of the bearings may be exposed to the hot and pressurized motive fluid vapors. Due to the exposure to the hot vapors, the metal temperature of a turbine shaft bearing is liable to become excessive. As a result of bearing overheating, metal or alloy based lining having good lubricating properties tends to become weakened and shears in the direction of shaft rotation. Without protection for the metal or alloy lining, the bearing surface geometry can become altered due to the metal-to-metal contact between the bearing and the shaft, ultimately leading to possible bearing failure and an unsupported turbine shaft. At times, the sheared metal or alloy blocks the oil inlet to the bearing, resulting in another cause of bearing failure.
In addition, the rotor between bearings design has been utilized only with respect to steam turbines. Thermal stress present in steam turbines is not similar to the thermal stress that may be present in organic vapor turbines.
It is an object of the present invention to provide turbine shaft bearing apparatus for use in a rotor between bearings design that is not subject to overheating.
It is an additional object of the present invention to provide turbine shaft bearing apparatus to facilitate an increase of the total power output of the turbine and of a power plant in which the turbine is incorporated.
It is an additional object of the present invention to provide a turbine module apparatus to facilitate an increase of the total power output of the turbine and of a power plant in which the turbine is incorporated.
It is an additional object of the present invention to provide turbine shaft bearing apparatus to facilitate efficient utilization of relatively low temperature heat sources or resources.
Other objects and advantages of the invention will become apparent as the description proceeds.